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Erosion helps keep mountains standing tall

26 June 2013

Uplifting erosion

(Image: Igor Shpilenok/NaturePL)

WHAT goes up must come down. So why are many mountain ranges, such as the Appalachians, still standing tall long after rivers should have eaten them down? Paradoxically, the erosive forces that should wear them away might instead carve them into a stable shape.

Recent research has it that rivers only chew through bedrock if their waters carry lots of abrasive sand and grit. When mountains are being forced upwards by tectonic processes, their slopes are generally steep and liable to collapse in landslides (see picture). These provide rivers with abrasive particles, further destabilising the mountainside and in turn causing more landslides.

Now David Egholm at Aarhus University in Denmark and colleagues have modelled the fate of mountains after the tectonic uplift is over. They found that mountain slopes soon erode to become more gentle and less likely to collapse. There are fewer landslides on mountains of that shape, so rivers carry less abrasive sediment and lose their ability to cut through the rock. Erosion no longer wears down the mountain, instead it largely grinds to a halt (Nature, DOI&colon; 10.1038/nature12218).

Existing models suggest that a 4-kilometre-tall mountain range would lose half of its height within 20 million years. Under Egholm’s team’s scenario, it would take more than 200 million years, which is closer to the age of many mountain ranges.